1. Field of the Invention
This invention relates to a lifting and guiding assembly for maintaining a rotary drive assembly, such as is used in mining and/or fluid thickening operations, in a desired orientation while vertically raising or lowering the rake drive assembly. Typically, these are used in liquid/solids sedimentation to raise and lower rotatable rake arms which engage settled solids within a tank and move the solids to a tank bottom outlet.
2. Statement of the Art
Vertical rotary drive shafts are generally fitted with lifting means capable of raising and lowering the drive shaft as required. Normally, the drive shaft is located in slidable engagement with a surrounding sleeve which serves to assist in maintaining the drive shaft in substantially vertical orientation. The sleeve is engaged with a motor so that, in operation, the motor rotates the sleeve. The sleeve and the drive shaft are connected so rotation of the sleeve also rotates the drive shaft. This connection is typically a sliding guide key arrangement by which drive torque is transmitted from the sleeve to the vertical drive shaft.
These types of devices have many uses in the mining industry and are also used in liquid/solids sedimentation assemblies. Thickeners or settling apparatus are used for solid-liquid separation of slimes, slurries and sludges. Such settling apparatus generally includes a relatively large settling tank and rotary rakes supported within the tank. The rotary rakes usually include rake arms positioned near the bottom of the tank and are driven to move settled material to a central outlet or to a tank periphery outlet.
It is well known that the vertical drive shaft connected to the rotary rakes is subjected to very high torques when rotated. The degree of torque experienced by the shaft is dependent upon the resistance to rotation experienced by distant portions of the shaft. This resistance arises primarily as a result of rake arms at the end of the shaft encountering resistance as they are rotated near the bottom of the tank through a liquid or through settled solids.
Additional stresses are placed upon the shaft when horizontal forces are also brought to bear on the shaft; for instance, when the drive shaft is deflected from its vertical position or where, in thickening operations, different densities of the sludge being raked by the rake arms place sideways pressures on the vertical shaft. The shaft also experiences vertical forces resulting from vertical lifting of the rake assembly in the tank (i.e., a dead load force).
These torque, horizontal and vertical forces on the drive shaft increase the friction between the shaft and the surrounding vertical sleeve in conventional systems. The amount of friction is proportional to such forces. When the drive shaft is lifted, the lifting means has to overcome these frictional forces in addition to the dead load force. This places additional strain on the lifting means.
Most thickeners are fitted with a lifting device which is capable of lifting the vertical shaft and rake arms above their normal operating position (i.e., with the rake arms positioned near the bottom of the tank) when it is desired to limit the torque applied to such vertical shaft and rakes. In this arrangement, the lifting device and the sliding guide key arrangement are adapted so that the drive shaft and rake arms may be lifted while the sleeve is still able to transmit drive torque to the drive shaft. Under high torque and/or high horizontal pressures, the frictional forces between the shaft and the sliding guide key arrangement may exceed the weight of the shaft and rake arm and require very powerful lifting devices (e.g. hoists, screw jacks or hydraulic cylinders) to lift the shaft and rake arm.
One type of thickener assembly is column-mounted. That is, it is generally secured on top of a rigid column. In such a thickener assembly, a rake drive unit is seated on the column, and includes a motor and gear boxes for rotating a cylindrical cage. The cage has one or more vertical legs connected to generally horizontal rake arms. One of the problems with conventional column-mounted thickener assemblies is that large frictional forces are also experienced as a result of the considerable torque forces imposed on the legs and rake arms as the rake arms rotate through the liquid or settled solids in the tank. Accordingly, when using conventional guiding means for maintaining the cage and legs in generally vertical orientation while raising or lowering them, the large frictional forces increase the amount of lifting force needed by the lifting means and a high powered lifting device is required. Another lifting device is disclosed in U.S. Pat. No. 4,624,787, which uses a pulley system to achieve the desired lift. This arrangement is complex and requires constant maintenance of the cable and pulleys.
With the above problems in mind, the present invention is directed towards improved guiding assembly mechanisms which optimally transfer torque forces, as well as horizontal forces and vertical (i.e., dead load) forces, from the rotating rake assembly and provide links which effectively lower the frictional forces encountered in conventional lifting mechanisms while providing simple linkage assemblies which avoid the need for employing heavy fabrication assemblies.